This project is designed to gain a better understanding of the in vivo control of the biosynthesis of deoxyribonucleotide triphosphate, the precursors of DNA. A key control point is in the reduction of ribonucleotides. An attempt will be made to determine the relationship between the synthesis of the four protein components of this system and the concentration of various possible corepressors (dTTP, dGTP, dCTP, dATP, and cysteine) by the use of various mutant strains. Also included in the proposed research is the isolation and characterization of mutants defective in the operator and regulatory genes cotrolling the ribonucleotide reductase system as well as mutants which produce a ribonucleotide diphosphate reductase with altered allosteric regulation. Mutants which produce a defective thioredoxin or a defective thioredoxin reductase will be isolated to determine the role of these proteins in metabolism other than in ribonucleotide reduction. A further objective is to isolate and characterize mutants defective in dUTP pyrophosphorylase and deoxynucleotide kinases to also determine their role in regulation of deoxyribonucleotide metabolism.